To pet is a verb describing the way people touch animals that they like, or to create a peaceful feeling—petting is gentle stroking.
Pet is the present tense form and petted is both the simple past and the past participle . . . at least, officially:
I pet my dog every day.
She petted her cat affectionately.
I have not petted my iguana today.
However, many native English speakers say pet for all three forms—especially in America—so much so that petted sounds strange and childish to some English speakers.
In formal writing, use petted for the past tense, as this is officially correct and normal. But, in conversation, you can use either pet or petted for the past tense without much fuss. Some people, however, will think you are incorrect no matter which way you say it.